This invention relates to an electronic circuit to be connected between a direct-current (d.c.) power supply and a load for converting one d.c. voltage into another preparatory to application to the load. More specifically the invention concerns improvements in such a d.c.-to-d.c. converter, particularly relating to a surge suppressor circuit, sometimes referred to as snubber circuit in the art, that is customarily incorporated with the converter.
Japanese Unexamined Utility Model Publication No. 1-127388 is hereby cited as describing and claiming a surge suppressor circuit in a d.c.-to-d.c. converter. A typical prior art d.c.-to-d.c. converter of this type (shown in FIG. 1 of the drawings attached hereto) includes a step-up or step-down transformer having a primary winding to be connected to a d.c. power supply via an electronic switching device, and a secondary winding connected via a rectifying and smoothing circuit to a pair of converter output terminals between which is to be connected a load to be powered. A voltage detector circuit is also connected between the pair of converter output terminals for detecting the converter output voltage. The output from the voltage detector circuit is applied to a control circuit for the switching device. The control circuit responds to the voltage detector output by making on-off control of the switching device accordingly, keeping constant the converter output voltage.
As taught by the Japanese unexamined utility model application above, the d.c.-to-d.c. converter includes the surge suppressor circuit comprising a diode, a capacitor and a resistor. The capacitor is connected in parallel with the transformer primary via the diode, and the resistor is. connected in parallel with the capacitor.
An inconvenience with the d.c.-to-d.c. converter of the kind in question is that a voltage surge occurs across the transformer primary when the switching device is opened while the transformer primary is carrying a current. Consequently, were it not for the surge suppressor circuit, the sum of the surge voltage and the supply voltage would be applied to the switching device, resulting in its destruction in the worst case. Actually, however, the surge voltage is taken up by the capacitor of the surge suppressor circuit.
A problem left unsolved in this type of d.c.-to-d.c. converter arises from the fact that the transformer primary has a leakage inductance and parasitic or stray capacitance, and that the switching device also has a stray capacitance. The result has been the creation of a ringing circuit that, in cases where the switching device takes the form of a field-effect transistor for example, has caused oscillation of the drain-source voltage of that switching transistor. The ringing frequency is so high that high-frequency noise has been generated which has interfered with the operations of other circuits.
Generally, the d.c. power supply of the converter takes the form of a rectifying and smoothing circuit connected to an alternating current power supply. For elimination of the ringing noise, a relatively high. impedance noise filter has had to be connected to the a.c. power supply, at the costs of the lower efficiency, greater bulk, and higher cost of the converter.